Stress and internal friction associated with light-induced structural changes of a-Si:H deposited on crystalline silicon microcantilevers

Abstract

The study of light-induced changes of the mechanical properties of a-Si:H should be valuable in the search for the microscopic mechanisms behind the Staebler–Wronski (SW) effect. We have developed a sensitive technique for studying such changes by depositing a-Si:H films onto commercial scanning probe microscope Si microcantilevers. The detection system of the microscope provides for measurements of beam bending, oscillation resonant frequency, and in-resonance damping factor. The internal friction of an a-Si:H film is much larger than that of crystalline Si and is the largest damping factor of the bilayer beam. We observed an increase in relative volume, Δ V/ V, with photocarrier generation rate, G, and exposure time, t, following ΔV/ V∝ G 0.7 t 0.45 in intrinsic as well as in 1 ppm PH 3/SiH 4 doped a-Si:H. The volume changes could be reversed by annealing and were the same for CW and pulsed light exposures using 400 μs long square pulses at a rate of 200 s −1. Based on the magnitude of Δ V/ V and the fact that it does not saturate we suggest that the structural changes causing Δ V/ V permeate the whole film and are not limited to defect sites.